ISDN can be defined as a network that provides end-to-end digital connectivity to support a wide range of services, including voice and non-voice service, to which users have access by a limited set of standard multi-purpose user-network interfaces. A digital connection from the main network to an end user is called integrated digital access. ISDN access is defined in terms of channels including B channel and D channel. The B channel transmits data at 64 kb/s for carrying user information such as voice and coded information or data information which is either circuit or packet switched. The D channel is 16 kb/s, primarily for carrying signaling information for circuit switching. The D channel may also carry packet switched information.
Typically, two types of access are provided. The first is basic rate access, which contains two (2) B channels and one (1) 16 kb/s D channel (2B+D) for a total of 144 kb/s. Each B channel can have a different directory number if required and both channels may carry voice or data up to 64 kb/s. The second type of access is primary rate access and typically includes thirty (30) B channels and one (1) 64 kb/s D channel (30B+D), which is typically used in a 2.048 Mb/s CEPT network. This access is mainly provided for connecting digital private branch exchanges (PBX) to the ISDN.
Access lines, e.g., telephone lines, provide a signal path between end users and a central office for carrying ISDN signals. At the central office, the access lines terminate at line cards, e.g., circuit boards, which provide physical terminations to the access lines, in addition to other features. The central office provides switching equipment, and other common equipment, used to connect the access lines to outgoing trunk lines of a communications network. The trunk lines are higher order transports than the access lines, such as a serial bus interface (SBI) provided as described in U.S. Pat. No. 5,060,229.
As described above, the access lines terminate in line cards which are carried in access line shelves. Each access line shelf comprises a plurality of line cards which are connected to the network trunk lines through a back plane of the access line shelf, and the common equipment and logic. During signal transmission between an access line and a trunk line, performance monitoring exists only between the common equipment in the central office and the network trunk lines. If a failure occurs in the signal transmission between the common equipment, the line card, and the access line, this failure may go undetected. This type of failure, referred to as a silent failure, is unacceptable, and therefore, a mechanism for determining that ISDN signal transmission failures are occurring is required.